Field of the Invention
The present invention relates to the monolithic integration of a laser and a modulator.
High bit rates in information transmission can only be achieved optically. In a transmitting unit, a single-mode laser (e.g. a DFB laser) is used as a light source and an electro-absorption modulator is used as a switch. In order to achieve the requisite performance specifications both in the laser and in the modulator, both components utilize quantum effects which occur in QW (quantum well) structures in the active layer. In the modulator, the QCSE (Quantum Confined Stark Effect) is utilized. Monolithic integration of the laser diode and of the modulator is desirable in many ways: the losses when injecting the radiation into the modulator are minimized; higher bit rates and less chirp (broadening of the spectrum during modulation) are facilitated; and the costs for the transmitting unit as a whole are reduced. One problem in that regard is that the energy bandgap of the QW structure of the modulator must be greater than the energy bandgap of the laser, so that no absorption of the radiation takes place in the modulator. For that reason, either different material compositions or different spatial dimensions of the layer structures have to be used in the laser and in the modulator.
There are various approaches for obtaining the requisite difference in the energy bandgaps of the laser and of the modulator:
1. Two epitaxy steps are carried out, in which the QW structure of the laser is grown first, it is then etched away on the modulator side and the QW structure of the modulator is grown in a second epitaxy step.
2. Selective epitaxy takes place: During the epitaxy process masks are used to control the growth conditions in different regions of the growing surface. That makes it possible to grow QW structures with different layer thicknesses.
3. Part of the surface is post-treated by ion implantation. That leads to misalignments between the lattice atoms, which are remedied by heat treatment. The result is to increase the bandgap, and that post-treated part can be used as a modulator.
4. A shared layer structure may be used for the layers and the modulator. That layer structure may be constructed as an MQW (Multiple Quantum Well) structure or as a superlattice. In an MQW structure, the potential wells provided by the layer structure are decoupled from one another. In a structure with a superlattice, the potential wells are coupled so that, because of the resonant tunnel effect, energy-level splitting takes place in the potential wells. In an MQW structure, the emission wavelength of the laser is tuned to red using a DFB grating, as described in European Patent Application 0 627 798 A1. When a superlattice is used, the Wannier-Stark-Localization effect occurs under the influence of an electrical field, so that the energy bandgap becomes greater (blue shift of the absorption edge). That is utilized in the laser modulator of European Patent 0 531 214 B1, corresponding to U.S. Pat. No. 5,305,343.